Generally, a cavity type wireless frequency filter (hereinafter abbreviated as a “filter”) has an accommodation space of a rectangular parallelepiped shape or the like through a metal housing, that is, a plurality of cavities, and for example, a dielectric resonant (DR) element or a resonant element made of a metal resonant rod is provided inside each of the plurality of cavities and thus a high-frequency resonance is generated. In some cases, a structure for generating a resonance with a shape of a cavity without having a DR element may be employed. Further, such a cavity type wireless frequency filter generally includes a cover for blocking an open surface of a corresponding cavity provided at an upper portion of a cavity structure, and as a tuning structure for tuning a filtering characteristic of the corresponding wireless frequency filter, a plurality of tuning screws and nuts for fixing the tuning screws may be installed at the cover. An example of a cavity type wireless frequency filter is disclosed in Korean Patent Laid-Open Application No. 10-2004-100084 (entitled “Radio Frequency Filter,” filed on Dec. 2, 2004, and Inventors: Park Jong-Kyu and two others) filed by the present Applicant.
Such a cavity type wireless frequency filter is used to process a transmission and reception radio signal in a wireless communication system, and specifically, the cavity type wireless frequency filter is typically applied to a base station or a repeater in a mobile communication system.
Recently, as a required data processing capacity increases in a mobile communication system, a proposal for installing a large number of small (or micro) base stations has been suggested so as to resolve a rapid increase of wireless data traffic. Further, technological development for weight reduction and miniaturization of equipment for processing wireless signals and installed in a base station is continuously underway. Particularly, since the cavity type filter requires a relatively large size due to a characteristic of a structure having a cavity, reduction in size and weight of such a cavity type filter has become a major consideration.
Meanwhile, important characteristics of the wireless frequency filter are an insertion loss and a skirt characteristic. The insertion loss refers to power which is lost while a signal passes through a filter, and the skirt characteristic refers to steepness of a pass band and a stop band of the filter. The insertion loss and the skirt characteristics have a tradeoff relationship with each other according to the number of stages (orders) of the filter. As the number of stages of the filter is increased, the skirt characteristic becomes better but the insertion loss becomes lower.
A method of forming a notch (an attenuation pole) is mainly used to improve a skirt characteristic of a filter without increasing the number of stages of the filter. A most common method for forming a notch is a cross-coupling method.
Generally, a notch structure of the cross-coupling method is mainly configured with a metal workpiece such as a metal rod which forms a capacitance coupling between resonant elements of two cavities which are not continuous in a circuit. The metal rod is installed to pass through an inner wall for separating the two cavities. At this point, in order to electrically isolate the metal rod from the inner wall, an outer portion of the metal rod is surrounded a support of a dielectric material (not shown) such as Teflon, and then is coupled to the inner wall. At this point, a portion at which the metal rod is installed at the inner wall may be formed with a through-hole structure. However, since a process for forming a through-hole at the inner wall is not easy, a portion of an upper end of the inner wall is generally cut and then a metal rod surrounded with the support is installed at the corresponding cut portion. The support serves as insulation of the metal rod as well as has a shape engaged with a shape of the cut portion of the inner wall and is fixed to a portion at which the metal rod is installed, such that the metal rod is fixedly supported.
U.S. Pat. No. 6,342,825 of K & L Microwave Co., (entitled “Bandpass Filter Having Tri-section,” Inventor: Rafi Hershtig, and Patented Date: Jan. 29, 2002), or U.S. Pat. No. 6,836,198 of RADIO FREQUENCY SYSTEMS (entitled “Adjustable Capacitive Coupling Structure,” Inventor: Bill Engst, and Patented date: Dec. 28, 2004) discloses an example of a technique for forming a notch using a cross-coupling method.
A notch structure using such a cross-coupling method may be almost indispensably applied to implementing a small or micro cavity type filter applied to a small or micro base station. At this point, due to space and size limitations resulting from a characteristic of the small filter, a distance between resonant elements and a metal rod should be designed to be very close so as to obtain a desired coupling amount in the notch structure using the cross-coupling method. However, it is very difficult to precisely implement a distance between the resonant elements and the metal rod to correspond to a required coupling amount with a tolerance in the range of, i.e., about ±0.03 to 0.05 mm, which is commonly used in metal processing, and thus deviation in cross-coupling amount between products becomes larger.
Accordingly, in the cross-coupling type notch structure applied to a small or micro filter, when implementing a designed structure as an actual product, it is required a very high processing accuracy when a cross-coupling type metal rod (and resonant elements) are manufactured and installed. For example, a machining tolerance of about 0.01 mm or less may be required in a gap between a metal rod and resonant elements. However, when a very precise machining tolerance is required, difficulty in machining operation is increased and a machining time is increased, and thus machining costs are increased and a production yield is lowered, such that there is a difficulty in mass production.